The present invention relates to a switched capacitor filter circuit, which constitutes an analog filter built in an LSI.
An LSI has been designed so as to have multi-function and complex configuration. In order to achieve function centralization and economization, a digital circuit and an analog circuit have been integrated on the same chip. A switched capacitor filter circuit is equivalent to an RC filter circuit comprising a resistor element (device) and a capacitor. However, the switched capacitor filter circuit receives almost no influence by un-uniformity resulting from manufacture process, and has a small area, as compared with the actual RC filter circuit; therefore, the switched capacitor filter circuit has been frequently used as an analog filter integrated into the LSI.
FIG. 1 is a circuit diagram showing a configuration of conventional switched capacitor filter circuit, and FIG. 2 is an equivalent circuit diagram thereof. The conventional switched capacitor filter circuit comprises three capacitors 11, 12 and 13, four switch circuits 14, 15, 16 and 17, and an amplifier 18. One electrode of the first capacitor 11 is connected to an input terminal 21 of the switched capacitor filter circuit via the first switch circuit 14.
The other electrode of the first capacitor 11 is connected to one input terminal of the amplifier 18 via the second switch circuit 15. The other input terminal of the amplifier 18 is grounded. An output terminal of the amplifier 18 is connected to an output terminal 22 of the switched capacitor filter circuit. Both electrodes of the second capacitor 12 are individually connected to the input and output terminals of the amplifier 18 via the third switch circuit 16 and the fourth switch circuit 17. The third capacitor 13 is connected between the input and output terminals of the amplifier 18.
The first to fourth switch circuits 14, 15, 16 and 17 periodically repeat an on/off operation based on a switching frequency f of signal input from a switching signal input terminal 23. By doing so, the charge and discharge of the first and second capacitors 11 and 12 are repeatedly carried out; therefore, the first and second capacitors 11 and 12 and the first to fourth switch circuits 14, 15, 16 and 17 function as equivalent resistor circuits 26 and 27 as shown in FIG. 2. In general, a cutoff frequency characteristic of the switched capacitor filter circuit is determined by a switching frequency f of the switch circuits 14, 15, 16 and 17 and each capacitance of three capacitors 11, 12 and 13.
However, the function centralization of LSI has been made; for this reason, there is the case where a frequency having plural different clocks is used in the circuit, and thereby, data processing must be carried out at plural kinds of transfer speeds. In such an LSI, there is the case where a signal called as beat having a frequency comprising a difference between plural frequencies is generated. For example, a beat frequency fb generated with respect to two frequencies f1 and f2 is expressed by the following equation (1) when m1 and m2 are set as integer.
fb=|m1xc3x97f1xe2x88x92m2xc3x97f2|xe2x80x83xe2x80x83(1)
As is evident from the equation (1), the beat frequency fb variously exists by the combination of m1 and m2. In this case, when a beat having a high signal level is generated in a filter band, this is a factor of deteriorating a filter characteristic. However, a signal level of each beat is different depending upon circuit configuration and layout; for this reason, it is difficult to previously estimate a beat level generable in a filter band, and to make a circuit design. Therefore, conventionally, in the case where characteristic deterioration by beat is generated, the LSI must be again designed; as a result, a problem arises such that the number of design processes increases.
It is an object of the present invention to provide a switched capacitor filter circuit, which can prevent filter characteristic deterioration by an influence of beat frequency without making a redesign of LSI by changing a beat frequency generated by plural different frequencies from the outside.
According to the present invention, a feedback capacitance of an amplifier is set to be variable, and the feedback capacitance is changed by a control signal from the outside, and further, a switching frequency of switch circuit, for switching the charge and discharge of capacitor is also changed in accordance with a change of the feedback capacitance.
When preventing a deduction of filter characteristic by the influence of beat frequency, the feedback capacitance of amplifier changes by the control signal from the outside, and the switching frequency of switch circuit for switching the charge and discharge of capacitor changes in accordance with a change of the feedback capacitance.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.